With her dodgy Irish accent Cait (pronounced "caught" not "Kate" in Ireland) can be one annoying companion. She makes references to Ireland in her dialog with you, so it's safe to assume she is from the Emerald Isle originally and is not a Boston Irish native.
As far as I'm aware, a Scottish voice artist was hired to do an Irish accent. It didn't work.
After you travel with Cait long enough and she trusts you, she will tell you about her history and give you the quest "Benign Intervention" which sets you upon the path to cure her addiction to chems.
Maxing Cait's approval will grant you the useful Trigger Rush perk, which increases action point regen when your health is below 25%.
This video looks at Cait's story in depth. If you haven't yet taken Cait as a companion, be aware that this video does contain spoilers about her story...